The present invention relates to a procedure for measuring large wheel angles, preferably front wheels, on steerable vehicles. The main requirement for achieving good running characteristics and slight tyre wear is for the driving wheels of the vehicle to assume specific angle positions with narrow tolerances. On a straight course, therefore, a certain toe-in occurs, which means that both wheels are directed slightly inwards in relation to the direction of travel. The wheels shall also have a certain camber in relation to a level road surface. When cornering both wheel axles are turned about individual kingpins which shall have a certain gradient in relation to the normal direction of a level road surface, both in the caster and in the banking (KPI) direction of the vehicle. Demands are also placed on the curve angle difference, i.e. the relationship of the drive angles of the two wheels to each other when cornering.
A large number of different measuring systems for the wheel angle have been developed to measure these angles. In most of them measuring devices are applied with the aid of holders on the wheels whose angles are to be measured. The measuring devices include measuring transducers for camber, toe-in and direction. The camber is normally measured as deviation from the vertical line and upon measuring it is assumed that the vehicle is parked on a level, horizontal surface. The rear axle is used when measuring toe-in and direction in the case of smaller vehicles such as private cars, whereas a reference line perpendicular to the chassis of the vehicle is preferred in the case of large vehicles. The measuring methods used are generally based on lines stretched between the wheels or on optical principles in which mirrors are applied on the rear wheels, or plates with markings are suspended at the front and rear ends of the vehicle chassis.
These devices are used to directly measure toe-in and camber. The kingpin banking is determined indirectly by measuring the influence of the kingpin gradients on camber at a few different course angles, preferably straight course, 20 degrees left-turning course and 20 degrees right-turning course. The banking of the kingpins is determined in the same way or by measuring how much a locked wheel is turned in direction of rotation when a left-turning or right-turning course is steered. The kingpin gradients are calculated from the camber or wheel turning values obtained, using known trigonometric formulae, either with the aid of ready reckoners or nowadays computer programs.
A limitation in most existing wheel measuring systems is the difficulty of measuring large course angles. This is of interest when measuring the curve angle difference and the maximum wheel deflection possible for a vehicle. The main problem is that the maximum wheel deflection in many vehicles is so large that the wheel hub with the measuring device applied disappears into the wheel housing, which makes both optical measurement and stretching a straight measuring string impossible. Furthermore, if the rear axle is used as a reference during measurement, the wheel blocks the view of the wheel forming the outer curve. One way of overcoming this problem is to place the ordinary measuring device on an extension shaft so that it outside the wheel housing. However, this entails a number of extra manipulations and also assumes that the design of the wheel housing allows sufficient space for such an extension. The method otherwise most often used hitherto to avoid this problem has been to use special, separate measuring devices for these large angles. Such a measuring device may consist of swivelling, graduated measuring plates placed under the wheels, or of a scale placed on the ground close to the wheel, a measuring beam from the measuring device on the wheel being directed obliquely down towards this scale. Characteristic of the measuring devices are that they require irritatingly many manipulations to be applied and calibrated on site or that, due to their complexity, they entail unreasonable extra expense. Consequently measuring large course angles is in practice often omittedxe2x80x94even in cases when it is called for.